lapdm: Track fn of primitives in struct lapdm_msg_ctx

This field will be used in follow-up commits to provide FN information
in RSLms primitives towars upper layers. This is needed for instance on
the MS side when a CCCH_DATA.ind is received containing a TBF ImmAss
with a relative FN indicating the Starting Time. Without tracking FN
advance, the uppers layers are not capable of figuring out the absolute
FN of the TBF Starting time.

The struct lapdm_msg_ctx is not really used outside of libosmocore, so
we are safe extending it.

Related: OS#3626
Change-Id: Icf986f4202703eb452bedc1b749bb8ce0c73706f
diff --git a/tests/lapd/lapd_test.c b/tests/lapd/lapd_test.c
index 739f9b7..2f2a7f4 100644
--- a/tests/lapd/lapd_test.c
+++ b/tests/lapd/lapd_test.c
@@ -184,6 +184,7 @@
 	/* LAPDm requires those... */
 	pp.u.data.chan_nr = 0;
 	pp.u.data.link_id = 0;
+	pp.u.data.fn = 0;
         /* feed into the LAPDm code of libosmogsm */
         rc = lapdm_phsap_up(&pp.oph, &chan->lapdm_dcch);
 	OSMO_ASSERT(rc == 0 || rc == -EBUSY);
@@ -206,6 +207,7 @@
 	/* LAPDm requires those... */
 	pp.u.data.chan_nr = 0;
 	pp.u.data.link_id = 0;
+	pp.u.data.fn = 0;
         /* feed into the LAPDm code of libosmogsm */
         rc = lapdm_phsap_up(&pp.oph, &chan->lapdm_dcch);
 	OSMO_ASSERT(rc == 0 || rc == -EBUSY);